Micro-electro-mechanical systems (MEMS) are likely candidates for extremely sensitive, inexpensive sensors, which can be mass produced. Microcantilever sensors offer much better sensitivities compared to other MEMS sensors and have surface areas of the order 10−4 cm2, which is smaller than that of other miniature devices (such as Surface Acoustic Wave devices, SAW) by orders of magnitude. They can be mass produced at relatively low cost using standard semiconductor manufacturing processes and have demonstrated superior detection sensitivities for physical, chemical and biological sensing. Microcantilever-based sensors have been shown to be extremely sensitive; however silicon or silicon nitride microcantilevers coated on one surface with gold do not have any particular chemical selectivity. Chemical selectivity has been achieved by coating the gold surface of the microcantilevers with a selective film such as a self-assembled monolayer (SAM) of an alkane thiol having a head group suitable for molecular recognition. Also, functionalized films can be attached to hydrogen terminated silicon and silicon nitride surfaces by photochemical hydrosilylation to achieve more stable coatings. The main feature distinguishing microcantilevers from other MEMS is their unique bending response. They have a high surface-to-volume ratio, and therefore changes in the Gibbs surface free energy induced by surface-analyte interactions lead to large surface forces. When such interactions are restricted to one surface, then the resulting differential stress leads to bending of the cantilever. This bending detection mode can be used in liquid phase, as well as in gas phase, which makes cantilever sensors suitable for both molecular and ionic analytes if selective adsorption can be achieved on one of their surfaces using analyte-specific surface functionalities. A preferred approach to the design of selective sensors is to immobilize agents for selective molecular recognition in a matrix that mimics the organic medium in a solvent extraction system. In this manner, the matrix can enhance both the separation and the achievement of chemical selectivity. The transduction part of the microcantilever sensor is based on binding the molecular recognition agent to one surface of the cantilever so that the adsorption-induced stress change can be detected via bending of the microcantilever.
A problem exists with the formation of SAM coatings on gold coated cantilevers if an array of cantilevers is used. It is difficult to apply a coating, especially if a long period of time is required for a tightly packed layer to form, to a single cantilever in an array without contaminating other cantilevers in the array. Other approaches to modifying a single surface of a silicon cantilever involve reaction of silane reagents with the Si—OH groups on the surface, but again it is problematic to modify only a single cantilever in an array. The photoactivation method of this invention provides a solution to this problem wherein cantilevers are only activated to react with an ethylene substituted hydrocarbon when irradiated with UV light.
Arrays of cantilevers can be conveniently prepared with each cantilever or group of cantilevers having a separate molecular recognition agent to impart chemical selectivity. Attachment of molecular recognition agents to the surface with robust Si—C bonds gives a layer with superior stability. For example, chromium(VI) or chromate can be selectively detected by the microcantilever of this invention.
Chromium is naturally occurring in several different oxidation states. The most frequently encountered forms are the III and VI oxidation states. Chromium(III) is an essential trace element in the human body and plays an important role in the metabolism of glucose, lipids, and proteins. In contrast, Cr(VI) in the form of chromate (CrO42−) is considered to be toxic to animals and humans. Most of the methods used to determine CrO42− (such as ion exchange, chromatography, and atomic absorption spectroscopy) are generally time-consuming, have less than desired accuracy, or are expensive.
In addition, Cr(VI) is more soluble in groundwater than Cr(III), and thus has a greater potential of affecting human health and the environment. Various techniques have been tested for the direct determination of Cr(VI) in water, but most techniques are not suitable due to insufficient detection limits and/or matrix interferences. The method that is widely being used at present requires selective reaction of Cr(VI) with 1,5-diphenylcarbohydrazide followed by spectrophotometry. The commercial sensor technique based on the above method and used widely for Cr(VI) monitoring, has a detection limit of ≈1.9×10−7 M, close to the current EPA regulation level of 2.1×10−7 M. However, this method is not viable for remote monitoring, and also would not be applicable if federal/local regulated levels are tightened. Therefore, developing inexpensive, easily deployable techniques with higher sensitivity is important for environmental monitoring and remediation. Due to the possibility of mass deployment at low cost, microelectromechanical systems (MEMS), especially microcantilevers, have attracted attention recently due to their high sensitivity of detection.